DESCRIPTION (Applicant's Abstract): The long-term goal of the proposed research is to understand the developmental processes that regulate organ formation. Current work focuses on the radial organization of the Arabidopsis root as a simple and tractable system. The strategy has been to identify and characterize genes that play key roles in regulating radial patterning through control of cell division and cell specification. A genetic approach was used to identify mutations in the SCARECROW (SCR) and SHORT-ROOT (SHR) genes which result in roots that lack one cell layer. Cloning of SHR and SCR revealed that they encode members of the GRAS family of putative transcription factors. SHR is upstream of SCR in a pathway which leads to transcriptional activation of SCR. Genetic and molecular evidence indicates that SHR acts non-cell autonomously to activate SCR and specify cell identity. The non-cell autonomous activity of SHR is evidence for a signaling center located in the internal cell layers of the Arabidopsis root which provides pattern information to the surrounding cell layers. The SHR and SCR genes will be used as molecular tools to address how the signaling center is established and functions, the nature of its signal(s), and which cells are competent to generate and respond to the signal(s). Alternative hypotheses as to the nature of the SHR-mediated signal will be tested. The first is that SHR encodes a transcription factor that activates a gene or genes which participate in a signal transduction pathway from the stele to the adjacent cell layers (indirect model). An alternative hypothesis is that the SHR gene product itself is a signal molecule and is able to translocate to the adjacent tissue from the stele (direct model). Conditional expression of SHR and SCR will be used to test the top-down model which postulates that radial patterning is initiated during embryogenesis and is propagated through continual signaling to the meristem. Genetic and genomic approaches will be used to identify new components of the signaling pathway controlling radial patterning. Specific aims are to: 1. Identify the nature of the SHR-mediated radial pattern signal: test direct and indirect models. 2. Characterize the competence and response of root and shoot cells to SHR-mediated signaling. 3. Determine the temporal requirements for SHR-mediated signaling: test the top-down model. 4. Identify new components of the SHR-SCR radial patterning pathway using genetic and genomic approaches.